Preventing CGA revisionist history

You are going to love my inital answer. Colour 6 is both dark yellow _and_ brown. As usual, I am going to look at the hardware side of things. I am also talking solely about true-blue IBM stuff. Let's start on the CGA card itself. I am going to assume text mode (about the only place you'll get colour 6 on a real CGA card), the colours are therefore determined by the attribute byte for each character. The attribute data from the vieao RAM is latechs in U35 ('273). The outputs of that go to a 4-bit 4-way multiplexer (U9, U10, '153), which, selects one set of 4 bits or the other depending on wheter that particular dot in the characer cell is set or not (OK, there's a lot more to it, blink mode, graphics modes, etc, but I don't want to have to explain all 6 pages of the schematic at this point). The outputs from that mux are latched again (U101, '174), then buffered and sent to the DE9 socket. So the card outputs the 4 bits _unchanged_ from the appropriate part of the attribute byte. And it's clear from the circuit that the bits are : Bit 0 : Blue Bit 1 : Green Bit 2 : Red Bit 3 : Intensity. So you could reasonably claim that 0110 was dark yellow and 1110 was light yellow. But IBM also include the following table (suitably re-formatted): I R G B 0 0 0 0 Black 0 0 0 1 Rlue 0 0 1 0 Green 0 0 1 1 Cyan 0 1 0 0 Red 0 1 0 1 Magenta 0 1 1 0 Brown 0 1 1 1 White 1 0 0 0 Grey (=='light black' ;-)) 1 0 0 1 Light Blue 1 0 1 0 Light Green 1 0 1 1 Light Cyan 1 1 0 0 Light Red 1 1 0 1 Light Magenta 1 1 1 0 Yellow 1 1 1 1 High Intensity White (=='light white') OK, so why the odd name for colour 0 1 1 0. Well, we now turn to the schematic of the CGA monitor. The 4 bits come in, and are buffered by 74S05 chips , Q201 and Q253 on my schematic. Now for the interesting bit. There are 4 of the open-collector inverters -- Q201d., Q201f, Q253e and Q253f -- whose outputs are connected together. This common signal will only be high if the input signals are (wait for it) I=0, R=1, G=1, B=0. That is, for this particular colour 6. For that colour, Q206 (a normal NPN transistor) is therefore turned on, and this will reduce the level of green in the final display. In other words this colour is a little redder than you might expect. I did say I'd only consider IBM stuff. But I will just mention that I've worked on clone monitors that used a '138 to decode the input signals and detect colour 6. I don't have any schematics to hand, though. And later monitors still often used a PROM to decode the input signals, possibly allowing several colour sets, and used 2 or 3 bit resistor DACs to feed the CRT driverr stages. Now what that PEOM contained, and whether colour 6 was still special, I don't know. So, to sum up : 1) IBM call that colour 'brown' in the CGA TechRef description 2) The CGA card doesn't do anything special with it, the obvious pattern of bits appears on the DE9 connector 3) But the monitor does. It contains circuitry to shift that colour a little towards the red. I don't think this is a historical accident. Hope that helps. -tony